Belt conveyors are used in many different industries to transport articles of all shapes and sizes. In some applications conveyors are called upon to receive, transport, and deposit relatively large and heavy pieces of material. In a mining operation for example, conveyors are often used to move bulk aggregate material such as rock, mineral, and ore.
Particularly at the loading station, these large articles, which in the case of mined rock take the form of large boulders, put correspondingly large stresses and strains on the belt conveyor assembly. At a typical loading station, the mined rock falls into a transfer hopper that directs the rock onto the conveyor belt. In a typical conveyor, the belt runs over idler rollers that are rigidly mounted to the support frame of the conveyor. Having no other outlet for dissipation of the energy, the impact shock of the falling boulders is transmitted directly to the belt, rollers, and structural components of the conveyor. This constant pounding at the loading point over time frequently leads to damage of these parts and their connections. Premature fatigue and failure of the conveyor system is frequently the result. This of course leads to costly down time of the mining operation, as well as a substantial expense just for repairs.
Thus, to minimize conveyor damage at the loading section and thereby reduce system downtime and reduce maintenance costs, an apparatus for effectively dissipating impact shock is needed. Further, the apparatus should be a simple design that can be easily retrofitted to existing conveyors with minimal effort.
Although various attempts have been made to address the problem of impact damage to conveyors, these designs have been only partially successful and have failed to provide a highly effective shock-absorbing suspension. Some designs have partially addressed the problem of impact damage by providing idler rollers made of a yielding material. In the Murphy U.S. Pat. No. 2,622,447 for example, the rollers are made of a resilient material such as rubber. Another alleged improvement places a thick resilient pad around the roller, or between the supports and the conveyor frame to absorb a portion of the impact shock. However, using a resilient roller, or the intermediate pad concepts provides only minimal shock absorption, and the substantial damaging forces that remain are transmitted directly to the rigidly-mounted support members. Further, where the resilient pad is placed under the belt, the increase in transport friction greatly increases the power requirement, and in addition causes deleterious belt wear.
Another group of designs have attempted to reduce impact shock by putting the rollers on a movable suspension. In a typical configuration, the rigid frame supporting the rollers rests on single springs that in turn are mounted to the parallel conveyor support beams. This single-spring design has several disadvantages. First, the stiffness of each spring must generally be very high to properly support one half of the impact loading. Thus, the control over the resiliency of the suspension is limited. This design also makes retrofitting expensive and difficult, as the brackets usually must be custom-made to fit the support beam spacing of the conveyor. Further, the suspension assembly for each set of rollers is prone to a lack of lateral stability.
Also, as the springs in these prior art designs are usually mounted on top of the conveyor beams. This increases the height of the conveyor in direct proportion to the height of the spring used. This, in turn, decreases the available headroom above the conveyor belt that is already limited in many mining environments. Additionally, many of these suspensions leave the spring exposed to dirt and debris and possible damage from contact with foreign objects. An example of a typical prior art suspension is U.S. Pat. No. 2,974,777 to Marsh.
Thus, as demonstrated by the deficiencies of the previous designs, there is a need identified for an enclosed conveyor suspension that provides controlled resiliency, increased stability, a minimal increase in conveyor elevation, and is relatively easy to retrofit to an existing conveyor.